Currently, traditional primary and secondary batteries are the primary sources of power for wireless sensors such as those that are used in the internet of things and other industrial and agricultural applications. These sensors, however, require charging or replacing the batteries with frequencies that range from daily to bi/tri-monthly for some ultra-low power sensors. In order to decrease the frequency of changing batteries, system capabilities are often compromised in order to operate at ultra-low power.
It also is known that electrochemical cells can vary their electrochemical equilibrium with temperature, a property referred to as the thermogalvanic effect. This can be expressed in terms of a temperature coefficient, a, given in units of volts per degrees kelvin (V/K). Electrochemical materials and redox couples with high temperature coefficients are used in thermogalvanic cells. See, e.g., Lee, Seok Woo, et al. “An electrochemical system for efficiently harvesting low-grade heat energy.” Nature communications 5 (2014): 3942, http://doi.org/10.1038/ncomms4942; Yang, Yuan, et al. “Charging-free electrochemical system for harvesting low-grade thermal energy.” Proceedings of the National Academy of Sciences 111.48 (2014): 17011-17016. http://doi.org/10.1073/pnas.1415097111; Agar, JN, Thermogalvanic Cells, Adv. Electrochem. Eng., 3, 31 (1964); all of which are incorporated herein by reference in their entireties. These cells require a temperature gradient in order to operate. But in the context of wireless sensors, it can be difficult to find places where sensors can both collect the desired data type and access a temperature gradient. In addition, the requirements of a temperature gradient often result in additional cooling fins, which can dramatically increase the size of sensor platforms, thereby decreasing their effectiveness and desirability.
Thermally Regenerative Electrochemical Cycle (TREC) cell systems have been demonstrated (by Lee above) to convert thermal energy directly into useful electrical work. But their arrangement requires the batteries to be charged and discharged at different temperatures in order to access the TREC effects. Using similar principles, Yang (cited above) demonstrated a charge-free system for power generation which takes advantage of cyclic temperature differences to create useful electrical work. However this system requires exotic and expensive materials (Prussian blue and ferro/ferricyanide). It also works using a single electrochemical cell.
Lithium-ion battery materials also undergo the Seebeck effect (i.e., a temperature difference between two dissimilar electrical materials produces a voltage difference between the two materials). Two materials with large Seebeck coefficient vs lithium (1.7 mVK−1) are LixCoO2 (LCO) at 2.1 mVK−1 and LixV2O5 (LVO) at 1.2 mVK−1. These cells can be used in a TREC arrangement to recapture heat energy.